1. Field of the Invention
The present invention relates to a display apparatus with a thin film transistor (abbreviated as “TFT” hereinafter).
2. Description of the Related Art
Recently, there have been a great demand for a portable display apparatuses such as, for example, portable television sets and portable phones, and, consequently, there is great demand to reduce the size, weight, and power consumption in these portable display apparatuses. Considerable effort has been devoted to satisfying this demand.
FIG. 1 is an equivalent circuit diagram of a conventional liquid crystal display apparatus.
As shown in FIG. 1, a liquid crystal display panel 100 includes a plurality of gate lines 51 connected to a gate driver 50 for supplying gate signals, a plurality of drain lines 61 to which data signals on data lines 62 are supplied when sampling transistors SPt1, SPt2, . . . SPtn are switched on in response to respective sampling pulses SP1, SP2, . . . , SPn output from a drain driver 60 for supplying drain signals, and an insulating substrate 10 on which the gate and drain lines are formed. A TFT 70 which is connected to a gate line and a drain line, and a pixel electrode 80 which is connected to the TFT 70 are provided near each of the cross sections between the gate lines 51 and the drain lines 61.
An external circuit board 90 is provided separately from the insulating substrate 10, and an LSI 91 for driving the panel is provided on the external circuit board 90.
Start signals for driving the panel are input to the gate driver 50 and the drain driver 60 from the LSI 91 on the external circuit board 90. Image signals are input on the data line 62.
A sampling transistor SPt is switched on in response to the sampling signal based on the start signal, and the data signal on the data line 62 is supplied to a drain line 61. Also, a gate signal is input from the gate line 51 to a gate electrode 13, and the TFT 70 is switched on. A drain signal is then simultaneously applied to the display electrode 80 via the TFT 70 and to a storage capacitor 85 for maintaining the voltage applied to the display electrode 80 for a duration of one field via the TFT 70. An electrode 86 of the storage capacitor 85 is connected to the source 11s of the TFT 70 and the other electrode of the storage capacitor 85, an electrode 87, is connected to a common voltage at each of the display pixels 200.
When the gate of the TFT 70 is opened and a drain signal is applied to the liquid crystal 21, the voltage of the signal must be maintained for the duration of one field. However, liquid crystal alone cannot hold the voltage, and the voltage declines as time passes. This reduction in voltage results in a flicker or an uneven display, causing a display degradation. The storage capacitor 85 maintains the voltage for the duration of one field.
When a voltage applied to the display electrode 80 is applied to the liquid crystal 21, the liquid crystal 21 aligns in response to the voltage and an image can be displayed. In this manner, a display can be obtained for both animated images and still images. In this case, voltages are applied to each of the LSI 91 on the external circuit board 90 and drivers 50 and 60 for driving each of the components. Power consumption therefore corresponds to these applied voltages.
Display of a still image on the display region comprising display pixels 200 of the liquid crystal display panel 100 as described above may be desired. For example, when the liquid crystal display panel 100 is to be used as a display section of a portable phone, on a portion of the display section a picture of a battery cell may be displayed as a still image indicating the amount of battery power remaining for the phone,.
When a conventional liquid crystal display panel is used, because the panel is driven regardless as to whether the displayed image is animated or still, the display on the liquid crystal display panel 100 is produced by driving the gate driver 50, drain driver 60, and external LSI 91 for driving the panel, even when displaying a still image.
Because of this, each of the drivers 50 and 60 and external LSI 91 constantly be consuming power, resulting in an overall increase in the power consumption of the liquid crystal display apparatus and reduction in the duration of usage time for cases of a portable phone having the liquid crystal display panel 100 with a limited amount of power supply such as a battery or the like.
In other words, the conventional apparatuses suffer from a disadvantage in that the same amount of power is constantly consumed when displaying a still image as when displaying an animated image.
A liquid crystal display apparatus with a static type memory at each display pixel is disclosed in Japanese Patent Laid-Open Publication No. Hei 8-194205 (JPA H08-194205, hereinafter referred to as the '205 publication). This liquid crystal display apparatus employs a memory in which a two-step inverter is positively feedbacked, that is, a static type memory, as a storing circuit for storing digital image signals in order to reduce the power consumption.
In this apparatus, as shown in, for example, FIG. 2 of the '205 publication, a static type memory element stores digital data and switching elements (transistors) are provided for each static type memory element. One of the terminals of the switching element is connected to a pixel electrode and a reference voltage Vref is supplied to the other terminal. The switching element receives the data stored in the memory element at its gate electrode to control the resistance value between the pixel electrode and the reference line which supplies Vref, and adjusts the bias condition of the liquid crystal layer.
However, with this configuration, when the switching element changes from the ON condition to the OFF condition, there is a possibility that the voltage of the pixel electrode becomes a fixed voltage and a direct current is applied to the liquid crystal layer. Because of this, there a refreshing operation must be performed at each of the transitions from ON condition to the OFF condition.
Even when the switching element is to be maintained in the OFF condition, when a leak current flows to the switching element, the voltage of the pixel electrode slowly approaches the reference voltage Vref due to the leak current, and uneven display may be generated due to the voltage change. In particular, for a still image, especially in cases, for example, where the remaining amount of battery is displayed on a portable phone, the period between consecutive write operation of the display data is quite long, and therefore, the amount of leak current becomes large. Thus, the uneven display becomes even a greater problem.
The conventional liquid crystal display apparatuses are suited for displaying a full color animated image corresponding to an analog image signal. Liquid crystal display apparatuses with a static type memory for storing digital image signals are, on the other hand, suited for reducing power consumption while displaying a still image with a low number of gradations.
However, because these liquid crystal display apparatuses have different image signal sources, simultaneous display of both full color animated images and a still image corresponding to the low power consumption on a single display apparatus is not possible.